In the related art, with regard to fluorescence observation apparatuses with which an affected site is diagnosed by using a fluorescent agent, there is a known method in which a region in a white-light image of biological tissue from which fluorescence is detected is displayed by replacing the region with a marker having a predetermined color. By doing so, an observer can reliably recognize the presence of the affected site in a viewing field. On the other hand, there is a problem in that it is not possible to observe the morphology of the affected site to which the marker has been applied. Furthermore, there is a problem in that a marker that is different in nature from the rest of the white-light image appears unnatural to the observer. In particular, by displaying, also with the marker, fine noise that is erroneously detected as fluorescence, the observer experiences a sense of flickering.
Thus, there is a known method in which a fluorescence image is displayed by being superimposed on a white-light image by allocating the fluorescence image in which fluorescence from the affected site is captured to one of R, G, and B component images that constitute the white-light image (for example, see Patent Literature 1). The superimposed image generated by this method includes information about the white-light image also in the region that corresponds to a fluorescence region in the fluorescence image. Therefore, it is possible to display the fluorescence region without hindering morphological observation of the affected site, and also, without giving the observer an unnatural impression or a sense of flickering. However, with the method of Patent Literature 1, in the case in which the fluorescence intensity is low relative to a bright return-light image like the white-light image, the visibility of the fluorescence region in the superimposed image may be poor.
Thus, there is a known method in which analog signals that express a fluorescence image and an illumination-light image, respectively, are obtained by receiving the fluorescence image and the illumination-light image by using a photoelectric conversion device and by subjecting these images to photoelectric conversion, and in which the analog signals representing the fluorescence image are amplified by a greater gain than the analog signals representing the illumination-light image (for example, see Patent Literature 2). The photoelectric conversion device and an AD conversion circuit that processes the analog signals generated by the photoelectric conversion device contain dynamic ranges. Therefore, in the case in which the fluorescence intensity is low relative to a bright illumination-light image, in some cases, it is difficult to ensure a sufficient gain for analog signals representing a fluorescence image to achieve a sufficient visibility of a fluorescence region in a superimposed image.
In addition, a means for displaying an image, such as a monitor or the like, also contains a dynamic range related to displaying. Therefore, in the case in which an illumination-light image is displayed on a monitor with a certain level of brightness, even if it were possible to ensure a sufficiently large gain for analog signals representing a fluorescence image, in some cases, it is not possible to display a fluorescence region with a sufficient brightness to ensure sufficient visibility of the fluorescence region in a superimposed image.